The proteins which participate in blood coagulation must assemble and function rapidly in order to prevent bleeding after trauma. A feature of coagulation which aids in efficiency is localization of enzymatic reactions to membranes. In the hemostatic process two membrane-bound enzyme complexes function sequentially to produce thrombin. Both membrane complexes require the rapid, high affinity membrane binding of a protein cofactor to localize the respective enzyme and initiate enzymatic function. The enzyme complexes are functionally related because the product of the first complex, tenase, is an enzyme which then assembles with a cofactor to form the second complex, prothrombinase. Although function of the isolated enzyme complexes has been characterized there is no mechanistic information to explain how the two enzyme complexes function coordinately. Specifically, the effect of either enzyme complex upon the function of the other is unknown and the mechanism of transfer of factor Xa from the tenase complex to the prothrombinase complex is unknown. Recently, I have shown that the two cofactor proteins, factor VIII and factor V, enhance the binding of one another to a membrane surface. Because factor V and factor VIII each enhance membrane binding of the other it is likely that they bind to each other. Binding of factor VIII to factor V may be the physical mechanism which places the two enzyme complexes adjacent to one another in order to efficiently transfer factor Xa to form the tenase enzyme complex to factor Va to form a prothrombinase enzyme complex. The proposed experiments will examine the mechanism whereby factor VIII and factor V bind rapidly to a membrane and the mobility of the bound proteins upon the membrane. In addition they will characterize the binding of factor VIII to factor V and define the enzymatic consequences of the binding interaction.